Grade 23 titanium (Ti6Al4VELI) is more-or-less a higher-purity evolution of grade 5 titanium (Ti6Al4V), building on the latter's already formidable qualities. Grade 23 titanium uses a lower amount of oxygen and iron compared to grade 5, providing it with top-notch mechanical properties such as link lower density and best-in-class corrosion resistance – all great things to have in a watch case that has to handle 300 meters of saltwater pressure, in addition to whatever else the deep throws at it.